R. Rossetti

Excited electronic states and optical spectra of ZnS and CdS crystallites in the ≊15 to 50 Å size range: Evolution from molecular to bulk semiconducting properties

Very small ZnS and CdS crystallites are made and stabilized in aqueous and methanolic media without organic surfactants. Low temperature (−77 °C) synthesis in methanol produces the smallest crystallites, ≈30 A diameter cubic CdS and <20 A diameter cubic ZnS. The crystallites are characterized by transmission electron microscopy and in situ optical spectroscopy (λ≳200 nm). The crystallites are too small to exhibit bulk band gaps in their optical spectra. In the band gap region, the small crystallites show a higher energy absorption threshold with a resolved spectral feature (quantum size exciton peak), not present in the spectra of larger crystals. The far ultraviolet spectra are unaffected by size at present resolution. These results can be understood in terms of the crystallite molecular orbitals, and an elementary confined electron and hole model.

Hybrid electronic properties between the molecular and solid state limits: Lead sulfide and silver halide crystallites

Tiny single PbS crystals of ∼25 A diameter are synthesized and studied optically in low‐temperature colloidal solutions. Electron microscopic examination shows a simple cubic rock salt structure with a lattice constant unchanged, within experimental error, from the bulk value. These crystallites lack the near infrared electronic absorption characteristic of bulk PbS. The small crystallite absorbance in the visible rises more steeply than does the bulk absorbance. These results reflect electron and hole localization if one considers the variation in effective mass across the band structure. A simple discussion of localization anywhere in the Brillouin zone is given. For the first time, crystallite syntheses are carried out in solvent mixtures that form transparent glasses upon cooling. The PbS spectra are independent of temperature (at current experimental resolution) down to 130 K, in contrast to earlier results for quantum size exciton peaks in ∼20 A ZnS crystallites. Previously published observations of...

Proton tunneling dynamics and an isotopically dependent equilibrium geometry in the lowest excited π–π* singlet state of tropolone

Fluorescence and fluorescence excitation spectra have been observed for tropolone isolated in neon matrices near 4 K. The fluorescence spectra are vibrationally analyzed showing that specific geometrical changes in the excited state are dependent upon isotopic substitution of the phenolic proton. The intramolecular tunneling barrier is found to be substantially higher in the ground electronic state. The gas phase data of Alves and Hollas are analyzed showing that the tunneling reduced mass is very close to that of a free proton (deuteron).

Time resolved energy transfer from electronically excited 3B3u pyrazine molecules to planar Ag and Au surfaces

The lifetime of 3B3u pyrazine near Ag surfaces has been measured down to a separation of ∼35 A, where it has decreased by a factor of ∼600 from the isolated molecule value. There is no evidence for saturation of the energy transfer rate. The technique and apparatus are described. Numerical solutions to the classical double interface theory of energy transfer show energy transfer rates should be sensitive to adsorbed gas on top of the excited molecule.

The mechanism of vibrational relaxation in solids: Multiphonon relaxation of O2(c 1Σ−u) in Ar, Kr, and mixed Ar–Kr matrices

The vibrational relaxation of 16–18O2(c 1Σ−u) has been directly time resolved in Ar, Kr, and mixed Ar–Kr matrices. A vibrational cascade (v=2→1→0) is produced following near resonant intersystem crossing from v=0 C 3Δu. The vibrational relaxation rates slow in Kr host despite the fact that O2 (c 1Σ−u) is more strongly solvated in Kr. In solid Kr, relaxation is slower than host induced fluorescence. The C 3Δu→c 1Σ−u intersystem crossing rate, the spectral shift, and the vibrational relaxation rates are monitored in mixed matrices as the environment changes from pure Ar to pure Kr. The spectral shift shows partial saturation, or weak complex formation, behavior; the vibrational relaxation behavior is consistent with pairwise additive forces. The relaxation rates are strongly temperature dependent. It is suggested that O2(c 1Σ−u), as well as other first row diatomics such as NO(a 4Π) and C−2(a 4Σ), relax via a direct multiphonon mechanism. An analogy with vibrational predissociation in gas phase van der Waal...

Structure and dynamics of the biphenyl ring torsion in solid neon and argon

The extent of phenyl ring torsional relaxation possible in low temperature frozen solvents has been investigated for biphenyl in solid neon and argon. In neon biphenyl is trapped in a twisted form, while in argon both twisted and planar forms occur. The twisted forms appear to undergo substantial movement in ϑ towards a planar form before excited state luminescence occurs. The planar form in argon undergoes a highly anharmonic, low frequency (∼8 cm−1), large amplitude ground electronic state torsional oscillation. The excited state splitting (571 cm−1) between the two phenyl rings is 177 cm−1 less than the value reported in crystalline biphenyl. The excited state vibronic structure is considerably different from that observed in crystalline biphenyl. The difference between torsional relaxation in soft solids and viscous flowing fluids, as well as the ability of soft solids to store elastic energy of compression, are discussed.

Ground and n–π* excited state structures of the hydrogen bonded complexes pyrazine⋅H2O and pyrazine⋅ (H2O)2

Vibronically resolved 1Ag→1B3u(n–π*) excitation and 3B3u(n–π*) →1Ag emission spectra are reported for isolated pyrazine, and the complexes pyraxine⋅Kr, pyrazine⋅H2O, and pyrazine⋅ (H2O)2 in solid neon host at 4.2°K. Distinct zero phonon lines appear in the complex spectra, enabling a separation of vertical and adiabatic excitation with respect to the low frequency intermolecular normal modes. In the monohydrate and bihydrate complexes, the hydrogen bond is only slightly weaker in the n–π* excited states. An upper limit for excited state hydrogen bond lengthening is obtained. The monohydrate 3n–π* complex decays purely radiatively, but a competitive intersystem crossing appears to occur in the bihydrate. The hydrate spectra and lifetimes are unchanged for D2O. Zero phonon lines are just resolved in the spectra of pyrazine isolated in solid ice at 10°K. These data enable a distinction to be drawn between specific solvation due to hydrogen bonding and general solvation due to a polar water environment.